It’s good to see positive discussion continuing on in this thread.
A bit more info regarding aircraft seating vs. car seating.
Of the various aircraft I have flown over the years, those that were of the transport aircraft category always had their restraint systems attached to the seats; a requirement for Part 25 certification. With regards to non-transport category aircraft (such as a Cessna 172), seat belt placement, though regulated, may be much less restrictive with respect to anchor locations, therefore, providing the manufacturer with greater leeway in seat belt design and placement; this, I would have to research to be sure of. However, with respect to Part 25 certification requirements, review of Advisory Circular 25.562-1B sheds some light on seat belt placement. In AC No: 25.562-1B, Dynamic Evaluation of Seat Restraint Systems and Occupant Protection on Transport Airplanes, APPENDIX 3. SEAT FAMILY DEFINITION, 11. Seatbelts and Anchors, a. Family of seat principles, it states “. . . . The seat belt typically consists of a latching mechanism, a belt anchor (which connects the belt to the seat) and webbing (which links the latch mechanism with the belt anchors). “ Subsection (3) further states “. . . . The seat belt anchor provides the load path between the belt anchor (part of the belt assembly) and the seat structure . . . . The seat belt anchor at similar locations within the seat assemblies must . . . . employ similar methods of attachment.” All of which alludes to the seat belt being fastened to the seat structure as a must for certification.
Mounting restraints to the seat structures of our cars should never be considered. In the event of a seat mount failure during and accident, rollover protection alone would be completely compromised. In transport aircraft category design and certification, attaching the restraints to the seat structure is predicated on experiencing high G loads during as an aircraft’s initial contact with the ground during a crash situation; certain assumed criteria regarding aircraft attitude and g-load at the time of impact applies here and can be researched in the link below for AC No: 25.562-1B, Dynamic Evaluation of Seat Restraint Systems and Occupant Protection on Transport Airplanes. Aircraft seat mounting structures (as well as some aircraft flooring) are designed to absorb high g-loads during such an occurrence while the restraints keep the occupants firmly (hopefully) in place. With seat mounts and some floor structures designed to fail in a crash situation, having the seat belts attached to the seat is the most viable option for crash survivability.
Immediately, interested parties such as myself who are seeking a safer seating solution in the event of a vertical impact, are up against two seat mounting challenges with the Lotus 7 design. First, and to reiterate what we all already know, is the seating position itself in these cars. The cockpit design (60 plus year old design) demands that the seats be located in a more upright position with approximately 15 degrees of incline. While this position, when augmented with properly installed and adequately utilized safety gear, is acceptable for an impact in a horizontal plane, it doesn't do anything to resolve the possibility of experiencing a vertical drop during an accident, such as that which happened to me. Being bound by inherent design demands that I (we?) must work with/around the seating position which may very well mean compromise to some extent as long as it lay within the acceptable limits of mitigated risk, be it personal or otherwise. In other words, what am I doing/what can I do/what risk am I willing to accept to lessening the chance of a repeat occurrence or a repeat injury? Obviously, decreasing the chance of a repeat occurrence is a whole other discussion. Decreasing the chance of a repeat injury has personally led me to the question of seating, the focus of my post.
The second challenge is seat mounting rigidity; so what's the compromise/tradeoff? When a driver straps his or herself into the seat, they want the harness cinched down tight (the objective, clearly, is to stay securely put in the event of a crash); I’m referring to a high performance driving environment when I say this. If we attempted to create a collapsible mounting structure, it may end up compromising the restraint’s ability to successfully keep us in place. So, we don’t want to necessarily decrease (at least not to the point it becomes detrimental) seat mounting rigidity, or at the very least, I don’t. We want (we need) to stay put. So, the dilemma is to come up with something that is somewhat crashworthy in both a horizontal and vertical plane. Right now, for me personally, it's the foam I mentioned; my samples arrived last week. Having said that, during the rebuilding of my car, seat mounting will be given the highest consideration. Here’s a thought; when I had my accident, my spine compressed a full one inch when the rear of the car impacted the ground…..take a guess at what happened to the overall tension of my shoulder restraints. Granted, if you place impact absorbing foam in your seat, you will a) raise your seating position by some degree, depending on what material you use and its overall thickness. Personally, I’m not out to break any records and am not the least bit concerned about moving my CG to a location that’s not even going to be easily measurable by any means, and I’m sure not going to build a car that places my seating position near vertical and akin to that of a formula driver. b) You will have to contend with the foam’s initial compressibility which is about 50% per the info I received from Janice at seatfoam. Granted, once you take a seat, sufficient time will have to elapse to allow the cushion to reach an equilibrium position (50% compression) in order to apply a true 1-g preload once strapped in. And the time it takes to reach this equilibrium will vary by weight, build (the size of your bum), temp, seat bottom design and foam type, thickness and layer preference.
In February, if I’d had two inches of the Confor Foam, one inch of medium and one inch of firm (my future preference at the moment), it would (after reaching equilibrium) have given me a full one inch of impact protection in the vertical plane. With hindsight being the proverbial 20-20, I’d much rather have had one inch of impact absorbing foam soaking up the G-loads (and a possibly slightly loosened lap belt that still kept me in the seat) than the one inch loss in vertebrae height I now have to live with for the rest of my life. Having had foam in my seat that day may have very well meant the difference in my walking out of the ER after a checkup vs. a 14 day hospital stint followed by what is now my 18th week of recovery; still on meds and still in the brace.
For those who would care to research aircraft seating, I’ve provided links below. The links for the FAR’s are repetitive with the Cornell links being an easier read than the .gov links….but just in case you prefer it from the horse’s mouth.
http://www.faa.gov/documentLibrary/medi ... 562-1b.pdfhttps://www.law.cornell.edu/cfr/text/14/25.785http://www.gpo.gov/fdsys/pkg/CFR-2015-t ... 25-785.pdfhttps://www.law.cornell.edu/cfr/text/14/25.561http://www.gpo.gov/fdsys/pkg/CFR-2015-t ... 25-561.pdfhttps://www.law.cornell.edu/cfr/text/14/25.562http://www.gpo.gov/fdsys/pkg/CFR-2015-t ... 25-562.pdf
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